1. Field of the Invention
The present invention relates to an endoscope system that projects light in different wavelength bands toward a target site of a test subject to acquire information about relevant objects, or objects of interest, existing in the target site. The present invention relates also to a display method for displaying images obtained by the endoscope system.
2. Description of the Related Art
Various kinds of spectral measurements have been applied to the medical and industrial fields. Endoscopy may be recited as a typical exemplar of the spectral measurement. As well-known in the art, an endoscope is configured to project illumination light from a distal end of a probing portion, which is introduced into a test subject, toward a target site inside the test subject, thereby to capture images of the target site.
Conventionally, a white light source like a xenon lamp or a metal halide lamp has been used as the illuminator of the endoscope. Recently, many methods of capturing images of a target site under illumination light of narrow wavelength bands (narrowband light) are in the spotlight as devices for making it easier to find out pathologies or lesions in the target site. Also such methods have been studied and developed that are intended to acquire information about densities or concentrations of light absorbing components, such as oxygen saturation of blood hemoglobin, or information about depths of blood vessels from the surface of the target site on the basis of image signals obtained under the illumination of narrowband light (see JPA2006-326153 and WO2007/116663).
According to a prior art disclosed in JPA2006-326153, because oxygenated hemoglobin and reduced hemoglobin have the same degree of absorbency to light of 805 nm, a ray of 780 nm (first wavelength) and a ray of 830 nm. (second wavelength) are projected toward a target site, the first and second wavelength being on opposite sides of the isosbestic wavelength of 805 nm. Then, differences in light volume of the reflected light from the target site are calculated between these rays of the first and second wavelengths. As for oxygenated hemoglobin, the difference will be a positive value. When hemoglobin is in an intermediate condition between oxygenated and reduced, the difference will be approximately zero. When hemoglobin is in the reduced condition, the difference will be a negative value. On the basis of the calculated differences, this prior art measures a value corresponding to the transitional oxygen binding rate of red blood cells that are moving in capillary vessels.
In a prior art disclosed in WO2007/116663, blue and green narrowband rays are projected toward a target site, and image signals obtained under these narrowband rays are subjected to a matrix operation such that the color of blood (an example of objects of interest that are relevant to the inspection) is converted into brown, for example, whereas the color of bile (an example of objects of no interest that are irrelevant to the inspection) is converted into magenta, for example. Thus blood or blood vessels and bile are shown in different colors in the captured image, preventing misperceiving bile as blood.
Because oxygen saturation of blood hemoglobin or blood vessel depth is not directly measured as an actual quantitative value but merely estimated from the image signals obtained under the illumination of narrowband light, the estimated value can be adversely affected by irrelevant objects which may exist in the target site but are irrelevant or extraneous for the inspection of relevant objects, i.e. blood or blood vessels in this case. That is, irrelevant objects may lower the adequacy and reliability of the obtained vascular information. The above-mentioned JPA2006-326153 does not disclose any measures for ensuring adequacy and reliability of vascular information against influence of the irrelevant objects.
Along with the demand for eliminating influence of irrelevant objects on the information about relevant objects like blood vessels, it becomes desirable for the operator of the endoscope or the doctor to be able to confirm that influence of the irrelevant objects is certainly eliminated from the obtained information. If the doctor is not sure of the obtained endoscopic information, he or she cannot effectively use the information for diagnosis. The prior art disclosed in WO2007/116663 merely discriminates the bile or object of no concern from the blood or object of concern by coloring them differently from each other. This prior art does not eliminate influence of the irrelevant objects.